1. Field of the Invention
The present invention relates to a printer which performs a recording operation by applying toner to a recording medium, such as printing paper, and more particularly, to an electrostatic ink-jet recording head used in an electrostatic ink-jet printer.
2. Description of the Related Art
Conventionally, printer recording technology based on non-impact systems has the advantage that the generation of noise during recording etc. is so small as to be negligible. Ink-jet recording systems, in particular, allow printing and recording at high speed directly onto a recording medium, using a simple construction. Furthermore, they also allow recording onto normal paper, and therefore represent extremely advantageous recording systems. For example, FIGS. 4-7 illustrate a conventional example of a recording head proposed as an ink-jet recording system. This conventional example uses an ink containing toner particles dispersed in a carrier liquid for printing onto recording paper 8. In addition to needle-shaped recording electrodes 3 provided on the recording head side, an opposing electrode 7 is also provided at the rear side of the recording paper in a position opposing the recording electrodes 3. In this system, an electric field is generated by applying a voltage to the recording electrodes 3 and the opposing electrode 7, and the toner particles in the ink are ejected towards the recording paper 8 by means of the electrostatic force created by the electric field.
As shown in FIG. 4, the ink-jet recording head comprises a substrate 1 made from an insulating material of plastic, or the like, and a base film 2 covering this substrate 1. The base film 2 is made from an insulating material, such as polyimide, and has a thickness of approximately 50 .mu.m. A plurality of recording electrodes 3 are patterned on the surface of this base film 2. The recording electrodes 3 are formed by plating a conductive material of copper (Cu), or the like, onto the surface of the base film 2 to a thickness of 20-30 .mu.m, and then patterning such that the interval between adjacent electrodes is 300 dpi pitch, namely, about 85 .mu.m.
The end portion of each recording electrode 3 projects externally (towards the opposing electrode) from one edge of the base film 2 by 80-500 .mu.m. The surface of the recording electrodes 3 is covered uniformly by a film of insulating coating material 4 to a thickness of 10 .mu.m or less, as shown in FIG. 5 and FIG. 6, which are enlargements of the portion indicated by arrow A in FIG. 4.
Furthermore, in the ink-jet recording head, a portion of the upper surface of the base film 2 is covered by a cover 5. The cover 5 is formed from an insulating material and is shaped such that it does not interfere with the projecting end portions of the recording electrodes 3. An ink supply inlet 5a and an ink drain outlet (not illustrated) are provided, respectively.
The space enclosed by the base film 2 and the cover 5 forms an ink chamber, and ink is introduced via the ink supply inlet 5a such that the ink 6 is always in a full state inside the chamber. A slit-shaped ink spray outlet 5b is formed at the edge of the cover 5, between the cover 5 and the base film 2. The aforementioned end portions of the recording electrodes 3 project through this ink spray outlet 5b. Thereby, an ink meniscus indicated by symbol 6a is formed at this slit-shaped ink spray outlet 5b.
A constant back-pressure is applied to the ink 6 in the ink chamber. Therefore, due to the surface tension and capillary action of the ink itself, the ink 6 forms an ink meniscus 6b having a concave shape at the ink spray outlet 5b. Since the end portions of the recording electrodes 3 project from the base film 2 and the cover 5, when viewed from above as in FIG. 5, the ink meniscus 6a forms a U-shape between adjacent recording electrodes 3. Furthermore, as shown in FIG. 6, when viewed from the side, the ink meniscus 6a has a downward concave shape.
Therefore, when a high-voltage pulse is supplied to one of the recording electrodes, the electric field concentrates on the end region of the ink meniscus 6a at the projecting end portion of that electrode. Induced by this electric field, the charged toner in the ink is expelled from the end region of the ink meniscus 6a. This forms an ink drop 6b, as shown in FIG. 5, which is ejected towards the recording paper 8 on the side of the opposing electrode 7 positioned opposite the recording head, and is thereby printed onto the recording paper 8.
FIG. 7 shows an approximate diagram of equipotential lines showing the potential generated between the recording electrodes 3 and the opposing electrode 40 during recording in a conventional ink-jet recording head.
When a voltage is supplied to a recording electrode 3, the equipotential lines in the vicinity of the projecting point 3a at the end of that recording electrode 3 assume a semi-elliptical shape surrounding the recording electrode 3, whose end portion is projecting from the ink spray outlet 5b. Furthermore, in PCT international publication (International Publication Number WO 93/11866), an invention is disclosed wherein conductive members projects towards an opposing electrode, and prescribed particles are caused to fly out from the ends of the conductive members by an electric field generated between these conductive members and the opposing electrode.
However, in the conventional ink-jet recording heads described above, there have the following kinds of problems. A first problem is that it is difficult to form the ink into a desired dot size when recording onto recording paper. This is because a high-voltage pulse is supplied to the recording electrode 3 as a recording voltage, and the end portion of the recording electrode 3 itself forms a discharge point 3a for the ink 6. In this process, there is insufficient electrostatic force acting on the toner particles near the discharge point 3a in the direction of the discharge point 3a.
In other words, as shown in FIG. 7, in the region surrounding the recording electrode 3, the equipotential lines 9 are virtually parallel to the direction of ink discharge, with the exception of the region in front of the discharge point 3a (opposing electrode side). Therefore, insufficient electrostatic force is generated in the direction of the discharge point 3a with respect to toner particles in the vicinity of the discharge point 3a. Since the electrostatic force acting on the toner particles is weak, the amount of toner particles supplied to the discharge point 3a is insufficient for forming the desired dot size.
A second problem is that the discharge of ink droplets becomes unstable. This is because the ink meniscus 6a connects continuously across the recording electrodes 3, having vertices at the discharge points 3a, and therefore, the liquid surface in the vicinity of a discharge point 3a which has discharged ink will vibrate and affect the ink meniscus 6a, thus making it impossible to obtain an ink meniscus 6a that is stable at all times. A third problem is the occurrence of ink droplet discharge faults due to excessive concentration of toner particles in the ink spray outlet 5b. The reason for this is that the ink spray outlet 5b in the cover which supplies ink 6 to the discharge points 3a for discharge, is formed in a portion of the ink chamber in the shape of a slit of a size which prevents overflowing of ink. Consequently, no flow of ink 6 is produced at the ink spray outlet 5b, and there is an excessive concentration of toner particles in this region, causing the ink viscosity to rise above the required level.